1. Field of the Invention
The present invention relates to a display apparatus used for a display terminal of general computer systems, microcomputer systems, and the like, and more particularly to a raster scan type display apparatus for displaying graphics and characters.
2. Description of the Prior Art
Recently, with the spread of computers and microcomputers, raster scan type display apparatuses using a cathode ray tube (CRT) or the like have become widely used for their display terminals. In the raster scan type display apparatus using a CRT or the like, a picture is displayed on the screen from the upper left-hand corner of the display screen by sequentially accessing display memory addresses.
Conventionally, the display apparatus using an ordinary raster scan type CRT comprises a timing generator, a display memory address generator, a display memory, a shift register, and a display monitor. The timing generator generates a horizontal clock, a horizontal synchronizing clock, and a vertical synchronizing clock. The display memory address generator generates a display memory address from the horizontal clock, horizontal synchronizing clock, and vertical synchronizing clock. Display data are read out from the display memory with the display memory address applied thereto and are converted from parallel form into serial form by a shift register to be outputted to the display monitor. The display memory address generator is composed of a horizontal counter which is reset by the horizontal synchronizing clock and counts the horizontal clock, and a vertical counter which presets a display start address stored in a vertical preset register address by the vertical synchronizing clock, and counts the horizontal synchronizing clock, and outputs the values of the horizontal counter and vertical counter as the display memory address to the display memory.
Operation of the above described conventional apparatus will be explained in the following. First, at a display start position (the upper left corner on the display screen) the horizontal counter is reset by the horizontal synchronizing clock, and the horizontal counter is preset with the display start address stored in the vertical preset register by the horizontal synchronizing clock. The horizontal counter counts up to a predetermined memory width during a horizontal scanning period. When the count value of the horizontal counter becomes a predetermined value corresponding to the predetermined memory width in one horizontal scanning period, the horizontal counter is again reset by the horizontal synchronizing clock and the vertical counter is counted up by one by the horizontal synchronizing clock. This process is sequentially repeated until the display position reaches a display end position (the lower right corner on the display screen).
When the display position has reached the display end position, the vertical counter is preset with the display start address stored in the vertical preset register by the vertical synchronizing clock, and the above scanning process is restarted.
In the above described arrangement, it will be possible only to vertically scroll the displayed picture by sequentially changing the value stored in the vertical preset register. However, the conventional, display apparatus cannot offer more complex display functions such as the panning display which freely displays any portions of a larger display memory, the split-screen display, the function that each of the split screens is freely arranged for graphic display and character display, and the window display.